1. Field of the Invention
The present invention relates to a matrix display with a device for compensating the coupling between rows and columns. It is used in optoelectronics in the production of liquid crystal displays more particularly used as converters of electrical information into optical information.
2. Discussion of Background
In per se known manner, matrix displays comprise a display cell constituted by two transparent insulating walls and by a material having a plurality of areas distributed in the form of matrixes and interposed between a first group of electrodes covering one of the two walls and defining p control rows and a second group of electrodes covering the other wall, constituted by parallel conductive strips and defining q control columns, line X.sub.i, in which i is an integer such as 1.ltoreq.i.ltoreq.p and column Y.sub.j, in which J is an integer such as 1.ltoreq.j.ltoreq.q, defining an area X.sub.i Y.sub.j of the material and having means making it possible to supply on the rows and columns appropriate excitation signals used for exciting an optical property of the material.
Numerous devices of this type are known for which the excitation is of an electrical nature and which e.g. use as the sensitive material a liquid crystal film. The invention can be applied with particular advantage to such devices, but it also applies in more general terms to any devices having a material, whereof an optical property can be modified with the aid of an electrical excitation. The material can be a liquid or solid, amorphous or crystalline body. The optical property can be an opacity, a refractive index, a transparency, an absorption, a diffusion, a diffraction, a convergence, a rotary power, a birefringence, an intensity reflected in a given solid angle, etc.
A known process for the control of such display means consists e.g. in the case of a liquid crystal cell in which the excitation is of an electrical nature, of applying to row X.sub.i, a periodic voltage V.sub.xi of mean value zero, whilst to the other rows is applied a zero voltage. To the columns Y.sub.j are applied periodic voltage V.sub.yj of zero mean value and of the same duration and frequency as voltage V.sub.xi, but which are phase-displaced relative thereto by a quantity .PHI..sub.ji. The value of this phase displacement .PHI..sub.ji is between 0.degree. for the signals V.sub.xi and V.sub.yj in phase and 180.degree. for signals V.sub.xi and V.sub.yj in phase opposition controls the grey level of point X.sub.i Y.sub.j.
In reality, the rising fronts of the addressing voltages V.sub.yj reaching the columns of a matrix display disturb, as a result of a capacitive effect, the voltages applied to the rows. In particular, an unselected line, which is normally subject to a zero voltage, is the seat of an intergering or parasitic voltage.
FIG. 1 shows the signals appearing on the electrode rows and the electrode columns. All the electrode rows are connected to earth, with the exception of row X.sub.i in which 1.ltoreq.i.ltoreq.p and to which is applied a voltage square wave, such as signal a. A periodic voltage is simultaneously applied to each column. The phase difference .PHI..sub.ij between signal a applied to row X.sub.i and the signal applied to column Y.sub.j determines the grey level of point X.sub.i Y.sub.j. To simplify the representation of the signals, the case has been assumed of the column signals either being in phase (.PHI..sub.ij =0), or in phase opposition (.PHI..sub.ij =180.degree.) with the signal a. This obviously does not constitute a limitation of the scope of the invention, which also covers cases in which the phase displacement .PHI..sub.ij is of a random nature. However, in such a case, the voltage at the terminals of cell X.sub.i Y.sub.j is more complex. Therefore, we will represent column signals which are only in phase or in phase opposition with the row signal. The column signals are consequently represented by signal b in phase with signal a, or by signal e in phase opposition with signal a. By capacitive effect, on the rows connected to earth, there appears an interfering signal, like signal c.
The original of this interfering signal is as follows. At a time t between 0 and T/2, in which T is the period of the signals applied to the electrodes, the voltages on the electrodes are constant and equal to +V or -V. On a column electrode subject to the voltage +V appears, in front of area X.sub.i Y.sub.j in which i is a row connected to earth, an electrical charge -Q and on row electrode i, facing area X.sub.i Y.sub.j, an electrical charge +Q. On a row electrode subject to a voltage -V, the electrical charge produced will be reversed, as will be the electrical charge appearing on row i. On writing .SIGMA..sub.Q, the algebraic sum of the electrical charges present on line i.SIGMA..sub.Q is not generally zero. It is not zero if there are the same number of column electrodes subject to a voltage +V as column electrodes subJect to a voltage -V. At a time t between T/2 and T, there is a charge -.SIGMA..sub.Q on row i in a symmetrical manner. At T/2, there is consequently a supply of -2.SIGMA..sub.Q electrical charges to row i and, as the display material is of an insulating nature, these charges are supplied by the row electrode i. Signal c translates the electric current no matter what the charge transfer. Signal c is in phase with the majority column signal, i.e. it is in phase with signal b if the majority of the column signals are in phase with signal a and it is in phase with signal e if the majority of the column signals are in phase opposition with signal a. In FIG. 1, a signal c is shown to be in phase with signal b. At the intersection of an unselected row and a column to which the signal b is applied, the potential difference applied to the material would be equal to signal b. However, due to the parasitic capacitive effect, the voltage applied at this point is represented by signal d which is equal to the difference between signals b and d. In the same way, the points belonging to an unselected line and whose column signal is represented by signal e are subject to a potential difference represented by signal f and equal to the difference between signals e and c, instead of being subject to a potential difference represented by signal e.
If there was no interference, a point of a row connected to earth and subject to a potential difference represented by signal b and a point of the same row subject to a potential difference represented by the signal e, would be subJect to the same excitation, because the area B of signal b is equal to area E of signal e. Due to the parasitic coupling between the rows and columns, signals b and e are respectively replaced by signals d and f. In the first case, the area below the curve of signal d equal to D is decreased, In the second case, the area beneath the curve of signal f and equal to F is increased. However, the excitation of the material varies like the square of the potential difference, i.e. like the area beneath the curve of the signal. Thus, this area difference leads to a parasitic visual phenomenon, such as lines on the screen.